Solid-state Raman lasers are a practical and efficient approach to optical frequency down conversion offering high (up to 70-80%) conversion efficiencies with respect to the pump laser power, excellent beam quality and ease of alignment In recent years use of crystals for stimulated Raman scattering (SRS) has been gaining interest because, in comparison with high-pressure gaseous and dye (liquid) Raman lasers, crystalline Raman lasers offer better gain, better thermal and mechanical properties, better reliability and the ability to operate at high pulse-repetition frequency. Further they are compatible with compact all-solid state laser technology.
The potential of crystalline Raman lasers for efficient frequency conversion was first reported by E. O. Ammann and C. D. Dekker, “0.9 W Raman oscillator”, J. Appl. Phys., vol. 48, no. 5, pp 1973-1975, 1977 who obtained 0.9 W at the first Stokes wavelength employing SRS in crystalline lithium iodate (LiIO3) inside the resonator of a Nd:YALO laser with optical to optical conversions of up to 77%. Other suitable crystals include various tungstates, molybdate, and barium nitrate. By selecting the nature of the crystal used it is possible for frequency conversion to particular discrete wavelengths.
The output beam at the Raman wavelength (or its second harmonic) typically has very good beam quality and spatial characteristics and pointing stability and can be efficiently (80-90%) coupled into optical fibres (typically 50-600 μm diameter) as required for many applications.
Solid-state Raman laser systems suffer from the problem however that they are complex in design and operation and it is generally difficult to design a solid-state Raman laser which is capable of being operated with stability from initial current input to the optical power pump source up to maximum current and over a long period of time. If the laser system is not effectively stable, alignment and power drifts result. It would be desirable to find a simple method to design solid-state lasers for a particular operating point within various power regimes including low power regimes (about 10 mW output power) and higher power regimes (greater than 1 W) which are able to be operated with stability. It would also be desirable to design a solid-state Raman laser system which is efficient, has low sensitivity to misalignment, good beam quality and can generate output of up to several Watts of Stokes output or when applicable, its second harmonic.